Process for producing cellulosic tubular films

ABSTRACT

The invention relates to a device for the production of cellulosic tubular films by extruding a solution of cellulose in an aqueous tertiary amine oxide into a precipitant present below the device, which device comprises an extrusion die having a substantially annular extrusion gap, wherein a supply duct for precipitant and a discharge duct for used precipitant are provided in the interior of the ring formed by the extrusion gap, characterized in that the supply duct ( 3 ) for the precipitant terminates below the discharge duct ( 4 ) for the used precipitant.

[0001] The present invention relates to a device for the production of cellulosic tubular films by extruding a solution of cellulose in an aqueous tertiary amine oxide into a precipitant present below the device, which device comprises an extrusion die having a substantially annular extrusion gap, wherein a supply duct for precipitant and a discharge duct for used precipitant are provided in the interior of the ring formed by the extrusion gap. The present invention also relates to a process for producing cellulosic tubular films.

[0002] From U.S. Pat. No. 2,179,181 it is known that tertiary amine oxides are capable of dissolving cellulose and that cellulosic molded bodies such as fibers may be obtained from such solutions by precipitation. A process for producing solutions of this kind is known, for instance, from EP-A-0 356 419. According to that publication, a suspension of cellulose in an aqueous tertiary amine oxide is initially prepared. The amine oxide contains up to 40% by mass of water. The aqueous cellulose suspension is heated and water is drawn off under a pressure decrease until the cellulose is dissolved.

[0003] For the production of cellulose fibers it is known from DE-A-28 44 163 to provide an air path or air gap between spinneret and precipitation bath in order to obtain a spinneret draft. That spinneret draft is necessary, since stretching of the threads is rendered very difficult after a contact of the formed spinning solution with the aqueous precipitation bath. The fiber structure adjusted in the air gap is fixed in the precipitation bath.

[0004] A process for producing cellulosic threads is, furthermore, known from DE-A-28 30 685, according to which a solution of cellulose in a tertiary amine oxide is molded to filaments in the hot state, the filaments are cooled with air and subsequently introduced into a precipitation bath in order to precipitate the dissolved cellulose. The surfaces of the as-spun threads, in addition, are wetted with water in order to reduce their tendency to adhere to neighboring threads.

[0005] A device for producing seamless tubular films is known from WO 93/13670. According to that known process, the cellulose solution is molded to a tube by an extrusion die having an annular extrusion gap, which tube is drawn over a cylindrical mandrel and introduced into a precipitation bath. In order that the extruded tube will not adhere to the surface of the mandrel, its surface is coated with a water film in a manner that the inner side of the tube coagulates and slides over the cylindrical mandrel. This has, however, the disadvantage that the water fed for wetting the surface of the mandrel can ascend as far as to the extrusion gap thereby wetting the die lip and causing the extrusion die to be cooled in addition to inducing undesired coagulations already during the extrusion procedure proper. This is undesired, because the cooled die cools the solution to be extruded, whose viscosity will thereby increase so as to render impossible a perfect extrusion to films having uniform thicknesses. Add to this that the known device is only cumbersome to reconvert if, for instance, films having different thicknesses are to be produced.

[0006] From EP-A-0 042 517 a process for producing a dialysis membrane of cellulose is known, in which membranes are produced from flat films or tubular films or hollow threads by means of appropriate dies.

[0007] From WO 95/35340 a blowing process for producing oriented cellulose films by spinning a cellulose solution in a precipitation bath is known, in which the solution is extruded through a film blowing die and an external air gap downwardly into the precipitation bath. The degree of orientation may be increased by longitudinal and transverse stretching.

[0008] From DE-A-195 15 137 a process for producing tubular films is known, according to which a cellulose solution at first is extruded to a tube, which tube on its way from the annular die outlet to the entry into the precipitation medium is stretched in the direction of extrusion and widened, i.e., extended at a ratio ranging between 1:1 and 1:10 by a gas pressure prevailing in the tube interior. By that extension, the tube is thus stretched transverse to the direction of extrusion.

[0009] A device of the initially defined kind, i.e., for producing cellulosic tubular films by extruding a solution of cellulose in a tertiary amine oxide into a precipitant present below the device, which device comprises an extrusion die having a substantially annular extrusion gap, wherein a feed duct for precipitant and a discharge duct for used precipitant are provided in the interior of the ring formed by the extrusion gap, may be taken from WO 95/07811. In that device, a spacer disc may be provided below the discharge duct in order to prevent the extruded tubular film from collapsing in the precipitation bath.

[0010] The prior art devices for the production of tubular films are complex in structure. It is, therefore, the object of the present invention to provide a device for the production of tubular films, which is of simpler construction.

[0011] The device according to the invention for producing cellulosic tubular films by extruding a solution of cellulose in a tertiary amine oxide into a precipitant present below the device, which device comprises an extrusion die having a substantially annular extrusion gap, wherein a supply duct for precipitant and a discharge duct for used precipitant are provided in the interior of the ring formed by the extrusion gap, is characterized in that the supply duct for the precipitant terminates below the discharge duct for the used precipitant.

[0012] It has been shown that in the device according to the invention the liquid level in the interior of the tube may be adjusted and, above all, kept constant in a simple manner. Furthermore, it has proved advantageous to the precipitating procedure, if the precipitant is allowed to stream opposite to the direction of extrusion, i.e., transport direction into the precipitation bath, in the interior of the tube. The present invention differs from the prior art also in this respect, since, as will be described below, fresh precipitant always gets into contact with film regions in which the cellulose is present in the precipitated state already to a high degree. On the other hand, the precipitant enriched most with amine oxide gets into contact with film regions in which the cellulose is present to a yet slightly precipitated extent.

[0013] A preferred configuration of the device according to the invention is characterized in that a spacer is provided below the extrusion gap, which spacer preferably is designed to have a substantially circular periphery.

[0014] The cross sectional area of the spacer may be larger than the area formed by the ring of the extrusion gap. This enables stretching of the tubularly extruded solution transverse to the transport direction.

[0015] The cross sectional area of the spacer may, however, also be smaller than the area formed by the ring of the extrusion gap.

[0016] The spacer preferably is designed such that the size of the cross sectional area may be changed. This is feasible, for instance, with a disc, or a ring, including displaceable members in a manner analogous to an apertured diaphragm, in which the displaceable parts may slide on or within one another, thereby enabling the external diameter to be changed.

[0017] A change in the size of the cross sectional area may, furthermore, be reached in that the spacer is comprised of an elastic ring, for instance of rubber, to which air, water or the like may be fed. The diameter of the elastic ring is, thus, controllable via the medium contained in its interior. A variant of this embodiment consists in that no complete ring is used, but the inner side is comprised of a solid part of, e.g., steel or plastics and an elastic part is attached to the same. Another embodiment consists in a semi-shell, optionally comprised of segments, whose fixed point is displaceable, thereby changing the external diameter (umbrella principle).

[0018] The invention also relates to a process for producing cellulosic tubular films, in which a solution of cellulose in a tertiary amine oxide is extruded through an extrusion die having a substantially annular extrusion gap, wherein the solution is molded in a tubular manner, which solution is transported into a precipitation bath and contacted with precipitant both on its inner side and on its outer side, whereby the cellulose is precipitated and the tubular film is formed, which is characterized in that the precipitant with which the tubularly molded solution is contacted on its inner side contains dissolved tertiary amine oxide whose concentration decreases in the transport direction of the tubularly molded solution.

[0019] In the process according to the invention, the tubularly molded solution may be stretched in the transport direction and/or transverse to the transport direction.

[0020] Stretching of the tubularly molded solution transverse to the transport direction may be effected by hydrostatic pressure or by gas pressure or by means of a spacer.

[0021] The tubular film molded by precipitating the cellulose after transportation through the precipitation bath is dried, wherein the tubular film is preferably kept under tension in order to prevent shrinking.

[0022] The process according to the invention is particularly suitable for processing aqueous solutions of cellulose in aqueous N-methylmorpholine-N-oxide (NMMO).

[0023] A preferred configuration of the device according to the invention will be explained in more detail by way of the annexed drawing.

[0024]FIG. 1 schematically shows a cross section through the lower part of an extrusion device substantially comprising an annular die 1 having an annular extrusion gap 2, a supply duct 3 for fresh precipitant (water or an amine oxide/water mixture) and a discharge duct 4 for used precipitant.

[0025] The cellulose solution is pressed through the extrusion gap 2 via the annular spinning mass space 5, thereby extruding the cellulose solution in the form of a tubular film 6 into the air space present between the precipitation bath surface 7 and the lower side 8 of the annular die 1.

[0026] The tubularly extruded solution 6 is drawn off into the precipitation bath 7, in which it gets into contact with precipitant on its outer side, thereby causing the dissolved cellulose to coagulate and the amine oxide to be released into the precipitation bath. Precipitant is also in the interior of the tube 6 such that the cellulose coagulates also on its inner side. In doing so, amine oxide is released again.

[0027] In the FIGURE, the level 8 of the precipitant present in the interior of the tube 6 is on equal height with the precipitation bath 7. Precipitant may be introduced into the interior of the tube 6 via the supply duct 3 and drawn off via the discharge duct 4. In this manner, the level 8 of the precipitant may be adjusted in the interior of the tube 6.

[0028] In the process according to the invention, the level 8 of the precipitant present in the interior of the tube 6 also may be adjusted to be higher or lower than the level of the precipitation bath 7. The composition of the precipitant in the interior of the tube 6 may differ from that of the precipitation bath.

[0029] The tubular film 6 is drawn off via deflection means 10 and thereby stretched in the transport direction, i.e., in the direction toward the precipitation bath 7.

[0030] The tubular film 6 is drawn over a spacer 9. That spacer 9 has the form of a circular disc firmly connected with the die 1 by means of rods 11. The spacer 9 comprises continuous bores 9 a for mass transfer. The supply duct 3 passes through the spacer 9. The disc also may be replaced with a spacer ring.

[0031] The tube 6 is extended by the spacer 9, this corresponding to stretching transverse to the transport direction. It is evident that such stretching transverse to the transport direction increases with the size of the circular spacer.

[0032] By feeding fresh precipitant via the supply duct 3 and discharging used, i.e. amine-oxide-containing, precipitation bath via the discharge duct 4, which terminates above the supply duct 3, the concentration of amine oxide increases in the direction toward the level 8 or, in other words decreases in the transport direction of the tubular film 6.

[0033] On the end of the supply duct 3 there may also be provided baffle plates (not illustrated) in order to divert the liquid stream from the supply duct 3 upwardly in the direction of the level 8.

[0034] Stretching transverse to the transport direction may also be obtained by means of hydrostatic pressure instead of the spacer. To this end, the level 8 of the precipitant within the tube 6 simply is adjusted to be higher than the level 7 of the precipitation bath. In doing so, extension is caused by the pressure of that part of the precipitant present in the interior of the tube 6, which is above the level 7.

[0035] Extension also may be obtained by means of gas pressure, wherein the die 1 must, of course, be provided with a gas feed (not shown), by means of which the tubular film 6 is inflated in the space provided between the lower side 8 of the die and the precipitation bath 7. The gas may also be replaced, which means that operation is effected at a gas excess. In that case, also a gas discharge duct (not illustrated), of course, is to be provided on the die. 

What we claim is:
 1. In a device for producing cellulosic tubular films by extruding a solution of cellulose in an aqueous tertiary amine oxide into a precipitant provided below said device, of the type including an extrusion die having a substantially annular extrusion gap constituting a ring, a supply duct for said precipitant and a discharge duct for used precipitant each provided in the interior of said ring formed by said extrusion gap, the improvement wherein said supply duct for said precipitant terminates below said discharge duct for used precipitant.
 2. A device as set forth in claim 1, further comprising a spacer provided below said extrusion gap.
 3. A device as set forth in claim 2, wherein said spacer is designed to have a substantially circular periphery.
 4. A device as set forth in claim 2, wherein said spacer has a cross sectional area larger than the area formed by said ring constituted by said extrusion gap.
 5. A device as set forth in claim 2, wherein said spacer is designed to have a cross sectional area capable of being changed.
 6. In a process for producing cellulosic tubular films, including the steps of preparing a solution of cellulose in an aqueous tertiary amine oxide, providing a device including an extrusion die having a substantially annular extrusion gap constituting a ring and a precipitant supply duct constructed to supply a precipitant as well as a used precipitant discharge duct constructed to discharge used precipitant each provided within said ring with said precipitant supply duct terminating below said used precipitant discharge duct, extruding said solution of cellulose in an aqueous tertiary amine oxide through said substantially annular extrusion gap of said extrusion die so as to obtain a tubularly molded solution, providing a precipitation bath containing said precipitant below said device, and transporting said tubularly molded solution in a transport direction into said precipitation bath so as to be contacted by said precipitant both on its inner side and on its outer side thereby causing said cellulose to precipitate and a tubular film to be formed, the improvement wherein said precipitant contacting said tubularly molded solution on its inner side contains dissolved tertiary amine oxide whose concentration decreases in said transport direction of said tubularly molded solution.
 7. A process as set forth in claim 6, wherein said tubularly molded solution is stretched in said transport direction.
 8. A process as set forth in claim 6, wherein said tubularly molded solution is stretched transverse to said transport direction.
 9. A process as set forth in claim 8, wherein said tubularly molded solution is stretched transverse to said transport direction by aid of hydrostatic pressure.
 10. A process as set forth in claim 8, wherein said tubularly molded solution is stretched transverse to said transport direction by aid of gas pressure.
 11. A process as set forth in claim 8, wherein a spacer is provided for stretching said tubularly molded solution transverse to said transport direction.
 12. A process as set forth in claim 6, further comprising drying said tubular film formed by precipitating said cellulose after transportation through said precipitation bath while keeping said tubular film under tension so as to prevent shrinking.
 13. A process as set forth in claim 6, wherein said tertiary amine oxide is N-methylmorpholine-N-oxide. 